1. Field of the Invention
This invention relates to a laser Doppler velocimeter (LDV) that is capable of operating either with a small focus diameter for analysing fluid flows at low velocity with high spatial resolution, or with a larger focus diameter to measure fluid flows at higher velocities accurately. More particularly, it relates to such an LDV in which a simple reversal of a lens pair will allow the LDV to operate in the two focus diameter modes.
2. Description of the Prior Art
LDVs have proven to be very valuable tools for measuring velocity characteristics of moving fluids, particularly for measuring air flow around air foils and similar structures in wind tunnels. The basic principle of operation is that coherent laser light scattered from particulate matter in a fluid moving with a particular velocity will be Doppler shifted by an amount determined by the laser wavelength and the index of refraction of the scattering medium. In the operation of an LDV, a plurality of laser beams are focused so that they have intersecting beam waists, which define a measurement volume at their intersection. As used herein, the term "waist" refers to the point of optimum fringe resolution along the laser beam. A single system having a pair of intersecting beams will provide one dimensional measurements. Conventional means can be used to collect and process the light scattered from such systems. Two subsystems disposed at an angle to one another and each having a pair of intersecting beams will provide measurements in two dimensions. Three subsystems disposed at different angles and each having a pair of intersecting beams will provide three dimensional measurements. Conventional means can be used to collect and process the light scattered from such systems. LDVs are described, for example, in the following issued U.S. patents: U.S. Pat. Nos. 3,825,346, issued July 23, 1974 to Rizzo; U.S. Pat. No. 3,860,342, issued Jan. 14, 1975 and U.S. Pat. No. 3,915,572, issued Oct. 28, 1975, both to Orloff; U.S. Pat. No. 4,148,585, issued Apr. 10, 1979 to Bargeron et al.; U.S. Pat. No. 4,263,002, issued Apr. 21, 1981 to Sathyakumar; U.S. Pat. No. 4,346,990, issued Aug. 31, 1982 to Rhodes; U.S. Pat. No. 4,470,696, issued Sept. 11, 1984 to Ballard; and U.S. Pat. No. 4,506,979, issued Mar. 26, 1985 to Rogers. The state of the art in LDVs is further indicated in Neti et al., "On-Axis Velocity Component Measurement with Laser Velocimeters", AIAA Journal, 17, No. 9, pp. 1013-1015, Sept. 1979.
While the LDVs described in the above prior art show a variety of optical systems used in LDVs, changing the focus diameter from a smaller to a larger size for measuring fluids flowing at higher velocities requires essentially tearing down the entire optical system and repositioning each of the components, from the last focusing lens up to and including the lasers themselves, in two or three dimensional systems.
In very high speed flows, the diameter of the beam waists in the sensitive volume (i.e., the intersection of the crossed laser beams) cannot be too small or measurement accuracy is lost. The scattering particle passes through a small sensitive volume in too short a time for proper measurement. However, at low speed, there is often a requirement for high spatial resolution, demanding small beam diameters at the sensitive volume. Thus, it would be advantageous to have an LDV system which could be easily changed between smaller and larger diameter beam waists in the sensitive volume.